This invention relates generally to missile seeker antennas and, more particularly, to a polarization compensator for such antennas which reduces the sensitivity of such antennas to radome-induced polarization errors.
As is known in the art of semiactive missile guidance, an illuminator may be used to direct a beam of electromagnetic energy toward a target (such as an aircraft), and a portion of such electromagnetic energy reflected from such a target back to a missile is intercepted and processed (here by using monopulse techniques) to derive guidance information to direct the missile to an intercept. For low altitude engagements, it is desirable to transmit vertically polarized electromagnetic energy and to utilize a vertically polarized seeker antenna in the missile in order to minimize the sensitivity of the seeker antenna to ground reflections by taking advantage of the fact that the reflectivity of the ground is less for vertically polarized electromagnetic energy than for horizontally polarized electromagnetic energy.
One known jamming technique based on transmitting cross-polarized signals from an aircraft is easily implemented to cause error in the guidance information derived in a missile. When the presence of a jamming signal of such nature is detected in the missile, a so-called "home-on-jam" mode of operation is initiated as a countermeasure to guide the missile toward the source of the jamming signal. In order that the home-on-jam mode of operation may be carried out with the greatest accuracy, provision must be made to compensate for the characteristics of the radome and the effect of such characteristics on monopulse techniques. Thus, if there were no radome, the response of the vertically polarized seeker antenna to a horizontally polarized jamming signal would be as much as 40 dB below the response to a vertically polarized signal and there would be no shift in the boresight axis of the antenna. However, in the presence of a radome having the shape of a conical ogive for aerodynamic purposes, a horizontally polarized wavefront experiences a degree of depolarization. Such depolarization produces a false, vertically polarized component which, in turn, results in a null in the antenna difference pattern which is displaced from the normal antenna boresight. It follows, then, that the vertical component created by the depolarization may cause angle tracking errors in the guidance information derived during the home-on-jam mode of operation against a jammer.
Radome-induced polarization errors during any mode of operation may be minimized by reducing the magnitude of the vertical component produced from a horizontally polarized jamming signal. One technique useful in reducing the magnitude of the vertical component produced from a horizontally polarized jamming signal or targets is to taper the sides of the radome perpendicular to the E-vector of the polarized energy, as is described in U.S. Pat. No. 3,314,070. While such a method may be effective in reducing the polarization errors experienced by signals in the plane of the tapers, the effectiveness of such tapers on signals incident on the intercardinal planes of the radome is not as pronounced. Another technique for the same purpose is to provide a radome having a thickness-to-wavelength ratio such that the transmission coefficients for both polarization components of a cross-polarized signal are very high and nearly equal. Such technique, however, requires a relatively heavy radome with a concomitant increase in weight of the missile.
Another technique useful in reducing the effect of any cross-polarized component in the echo signal received by a radar system is described in U.S. Pat. No. 3,805,268. Such a technique involves the use of a polarization-sensitive phase shifter applied to selected portions of the aperture of a reflecting antenna so as to produce similar primary and cross-polarized radiation patterns. Such a technique is, however, not well suited for use when the receiver antenna is a planar array.